1. Field of the Invention
The invention relates to a power steering system.
2. Description of Related Art
There is a conventional power steering system including both an electric steering assist device that uses a motor as a power source and a hydraulic steering assist device that uses an internal combustion engine, which serves as a vehicle axle driving source, as a power source, in order to apply assist torque to a steering system of a vehicle, as described in, for example, Japanese Patent No. 5034446.
The electric steering assist device is a column assist-type steering assist device. The electric steering assist device produces output torque by adding assist torque, which is generated by the motor, to input torque, which is input into a steering wheel, and then applies the output torque to the hydraulic steering assist device. The hydraulic steering assist device is a rack-and-pinion-type steering assist device including a rotary control valve. Upon application of the output torque from the electric steering assist device, the control valve is operated. Through the operation of the control valve, the flow path and the flow rate of hydraulic fluid that is supplied from a hydraulic pump to a power cylinder disposed in a rack body are switched based on a torsion angle of a torsion bar incorporated in the control valve. The power cylinder generates a rack driving force (steering assist force) as the hydraulic fluid is supplied to a cylinder chamber for right steering assist or a cylinder chamber for left steering assist. A solenoid relief valve is disposed between a high-pressure flow path that leads to the hydraulic pump and a low-pressure flow path that leads to a reservoir tank.
At normal times, the electric steering assist device does not generate assist torque using the motor when the input torque is lower than a set value. That is, the input torque is used as it is, as the output torque from the electric steering assist device. Assist torque corresponding to the output torque is generated by the hydraulic steering assist device until the output torque reaches the set value. After the output torque reaches the set value, the electric steering assist device starts generating assist torque, while the solenoid relief valve is opened and thus part of the hydraulic fluid is returned to the reservoir tank, so that the assist torque that is generated by the hydraulic steering assist device is limited. In this case, the output torque produced by the electric steering assist device is a value that is obtained by adding the assist torque generated by the motor to the input torque input into the steering wheel.
If one of the electric steering assist device and the hydraulic steering assist device malfunctions, steering assist is continued by the other one of the hydraulic steering assist device and the electric steering assist device, which is operating properly. If the electric steering assist device malfunctions, a set valve opening pressure, at which the solenoid relief valve is opened, is changed to a value higher than that used at normal times. Because it is possible to increase the flow rate of the hydraulic fluid that is supplied to the power cylinder to a flow rate higher than that used at normal times, the hydraulic steering assist device is able to generate assist torque corresponding to the torque applied to the hydraulic steering assist device not only when the torque applied to the hydraulic steering assist device is lower than the set value but also when the torque applied thereto is equal to or higher than the set value. That is, if the electric steering assist device malfunctions, a shortfall in the assist torque due to the malfunctioning is covered by the steering assist force generated the hydraulic steering assist device.
If the hydraulic steering assist device malfunctions, the electric steering assist device generates assist torque using the motor even when the input torque input into the steering wheel is lower than the set value.
In the power steering system according to Japanese Patent No. 5034446, the control valve of the hydraulic steering assist device is operated based on the output torque from the electric steering assist device. The stiffness (spring constant) of the torsion bar of the control valve is set based on the output torque from the electric steering assist device. That is, the torsion bar is appropriately twisted upon application of the output torque from the electric steering assist device operating properly. As a result, desired operation characteristics of the control valve are obtained. Therefore, if the electric steering assist device malfunctions, a steering feel similar to that before occurrence of the malfunction is obtained until the input torque input into the steering wheel reaches the set value. However, in order to make the hydraulic steering assist device deliver sufficiently high performance when the input torque exceeds the set value, the input torque needs to be higher than that at normal times because assist torque from the motor is not obtained. In addition, there is a possibility that the opening degree of the control valve and thus the steering assist force due to hydraulic pressure will not be sufficiently obtained because high input torque is required. This leads to deterioration of the steering feel.